<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <title>Document</title>
</head>

<body>

    <script type="text/javascript">
        /*
      基于Promise发送Ajax请求
    */
        function queryData(url) {
            var p = new Promise(function (resolve, reject) {
                var xhr = new XMLHttpRequest();
                xhr.onreadystatechange = function () {
                    if (xhr.readyState != 4) return;
                    if (xhr.readyState == 4 && xhr.status == 200) {
                        // 处理正常的情况
                        resolve(xhr.responseText);
                    } else {
                        // 处理异常情况
                        reject('服务器错误');
                    }
                };
                xhr.open('get', url);
                xhr.send(null);
            });
            return p;
        }
        // queryData('http://localhost:3000/data')
        //   .then(function(data){
        //     console.log(data);
        //   },function(info){
        //     console.log(info)
        //   });
        // ============================
        // 发送多个ajax请求并且保证顺序
        queryData('http://localhost:3000/data')
            .then(function (data) {
                console.log(data)
                return queryData('http://localhost:3000/data1');
            })
            .then(function (data) {
                console.log(data);
                return queryData('http://localhost:3000/data2');
            })
            .then(function (data) {
                console.log(data)
            });
    </script>
</body>

</html>